Scientists surveying a nature reserve in South Africa have discovered 18 previously unrecorded species of invertebrates, including spiders, snails, millipedes, earthworms and centipedes.

The trove of creatures was uncovered in eight days by researchers and volunteers working for the environmental charity Earthwatch at the Mkhambathi nature reserve on the spectacular Wild Coast in the Eastern Cape.

However, scientists warned that planned developments in the area could threaten the ecosystem and deny them the chance to identify further species.

Jan Venter, an ecologist working for Eastern Cape Parks, which manages the reserve, said that the 29 square mile area had previously attracted only ad hoc surveys and butterfly collectors.

"To get so many species in one survey shows the importance of the reserve. It's a very special area, conservation-wise. If we do another survey, we'll find just as many." The team suspects that another 18 species might be discovered.

The number of identified species on the planet – 2m so far – could represent only 2% of all those that exist. South Africa, one of the most biodiverse countries in the world, and its neighbours are prime locations for adding to the species catalogue.

Michelle Hamer, a scientist at the South African National Biodiversity Institute, said: "These discoveries are important because they highlight just how little we know about our biodiversity, even in a relatively well-studied country like South Africa … many of the species we collected seem to be unique to a small area in or around Mkhambathi."

The area has been earmarked for a toll road and titanium mining by an Australian company, though the development is said to be on hold due to the recession.

Hamer warned: "There is also a lot of pressure to develop tourism infrastructure inside the reserve. This means that many of these species could have disappeared before even being discovered. If we understand the importance of the area in terms of its invertebrate fauna, then we can try to protect it.

"Will it make a difference … if these species go extinct? We don't know for sure, but we do know that every species that is removed … results in some weakening of the ecosystem."

A report highlighting the data on the invertebrates at Mkhambathi was this year presented to Eastern Cape Parks scientists for use in conservation management. The freshly discovered species are now being named and described in South African research institutes.